At present, one of the essential problems faced be welding and metallurgical technologies is enhancing the stability of electrodes of electric arc apparatus operated for welding and similar processes of treatment of metals, as well as for melting and similar processes of treatment of metal and slag melts in electric-arc furnaces.
Known in the art is a method of electric arc treatment of materials (U.S. Pat. No. 3,307,011, Cl. 219-74, 1963), providing for enhancing the stability of electrodes, graphite electrodes included, of electric arc apparatus. The method includes introducing into the space between the electrodes of an electric arc apparatus plasma-forming mixtures including carbon-containing compounds chosen from the class of hydrocarbons and carbon monoxide, and gases inert with respect to the material of the electrodes. With the electric arc burning, these carbon-containing gases decompose and release free carbon which is deposited on at least one of the electrodes. Used as gases inert with respect to particular materials are argon or helium when the electrode is made of graphite, or else air or nitrogen when the electrode is made of copper.
This method makes it possible to reduce the erosion of electrodes and thus to extent their operating life with stable arcing at currents from 400 to 1000 amperes.
However, this process would not ensure an equilibrium of the respective quantities of the loss of carbon and its deposition on the working surface of the electrode. The weight of the electrode would either continuously diminish, although at a small rate, which means that the electrode erodes, or else continuously grows, which means that the dimensions of the electrode increase. In the first-mentioned case the electrode becomes ultimately destroyed. In the second-mentioned case the stability of arcing is affected on account of hindered localization of the electrode-adjoining region of the arc on the increased working surface of the electrode.
The aforementioned phenomenon can be explained by the following reasons: firstly, the use of only the gases inert with respect to the material of the electrodes in the composition of a plasma-forming mixture; secondly, the lack of quantitative relationship between the arc current value, the composition and flow rate of the plasma-forming mixture; and thirdly, the lack of relationship between the variation of the composition of the plasma-forming mixture and the time of arcing, particularly, in the initial period of ignition of the arc. The necessity of using electrodes which are either block-shaped or hollow in some cases restrains the applicability of the method, as it would not ensure the required localization of either the arc or the jet of plasma within the treatment area.
It is for these reasons that the aforementioned method has not yet found industrial applications notwithstanding the fact that it was published as many as twenty years ago.
There is further known a method of electric arc or plasma treatment of materials (U.S. Pat. No. 4,317,984, Cl. 219-75, 1982; FR, B, No. 2431240, Int. Cl..sup.3 HO5H 1/48, B23K 9/16, 1983) consisting of introducing into the space between the electrodes of an electric arc apparatus a plasma-forming mixture including carbon-containing compounds and an oxidizing agent.
The oxidizing agent is introduced within a time interval corresponding to the reduction of the flow of heat into the electrode on which carbon is deposited from the peak value to a steady value. Depending on the nature of the oxidizing agent, its content in the plasma-forming mixture is taken to be either 0.4 to 0.9 or 1.05 to 2.5 by volume of that corresponding to theoretically complete conversion of hydrocarbons in the mixture. The first-mentioned range applies to an oxidizing agent in the form of either oxygen or air, and the second-mentioned range applies to an oxidizing agent in the form of carbon dioxide. Depending on the chemical affinity of the oxidizing agent for the carbon of the carbon-containing compound, the latter is supplied into the space between the electrodes in the amount of from (0.5.times.10.sup.-3)/n to (6.times.10.sup.-3)/n l/amp-sec, where "n" is a number of atoms of carbon in the employed carbon-containing compound. The carbon-containing compound and oxidizing agent may be introduced into the space between the electrodes either separately or jointly. When the heat flow into the electrode on which carbon is deposited attains the peak value already at the moment of striking an arc, the oxidizing agent is introduced into the space between the electrodes jointly with the carbon-containing gas prior to striking an arc.
This method provides for performance of the electrode in an equilibrium of the loss of carbon therefrom and supply of carbon thereto, i.e. in a mode of constant restoration of the electrode from the plasma-forming medium.
However, this method determines all the conditions for ensuring the operation of the electrode in a mode of constant restoration only on the part of the plasma-forming medium, and does not deal with similar conditions on the part of the electrode itself. This has been found to impede the reproducibility of a mode of constant restoration of the electrode when the value of the current of the arc varied within a relatively broad range, resulting in erosion of the electrode.